Problem: On the first 5 tests of his math class, Ishaan got an average score of 87. What does he need on the next test to have an overall average of 89?
Let his score on the next test be $x$ The sum of all of his scores is then $5 \cdot 87 + x$ The same sum must also be equal to $6 \cdot 89$ Solve: $x = 6 \cdot 89 - 5 \cdot 87 = 99$.